The generated tree is swept rather than deleted, which puts a delete on
the build's critical path with only an end-to-end comparison behind it.
Fourteen tests hold the three behaviours that matter: front matter is
recognised and stripped for the body comparison but not for the
decoration's own, a rebuild on unchanged inputs writes nothing and keeps
its front matter, and the sweep removes a page only when nothing
produced it. Reverting any of the three fails at least two of them.
